1. Field of the Invention
The present invention relates to a display signal processing device for processing a television signal and a non-television signal such as a graphic signal, and an LED display system incorporating the same.
2. Description of the Related Art
In recent years, full color LED display systems have been spreading rapidly in the field of indoor and outdoor information displays. Conventionally, such display systems often employ NTSC (National Television System Committee) video signals in accordance with the NTSC system. The NTSC video signals can be obtained from various video sources, such as televisions, VCRs, laser disk players, video cameras, and the like. Moreover, the demand for such display systems for use in a high resolution video display based on the HDTV (High Definition Television) standard is expected to increase in the near future.
Hereinafter, an exemplary structure of a conventional display system will be described with reference to FIG. 9.
In FIG. 9, the display system includes a transmission section 19 and a display section 20. The transmission section 19 includes: AV equipment such as a VCR 3, TV tuner 2, laser disk player 50 and video camera 4; an AV selector 5; a scheduler 9 for managing broadcast contents; a telopper 48 for processing videos; an amplifier 6; and a video signal transmitter 49 for long distance transmission of NTSC video signals. The display section 20 includes: a video signal receiver 113 for receiving NTSC video signals and converting the received signals to digital RGB (R: red, G: Green, B: blue) signals; a controller 14 for converting the digital RGB signals to signals suitable for an LED display section 8; an LED panel display 17; a power source 16; a loudspeaker 15; and a housing 18.
The operation of the display system having such a structure will be described.
The AV equipments in the transmission section 19 including a VCR 3, TV tuner 2, laser disk player 50 and video camera 4 are connected to the inputs of the AV selector 5. The AV selector 5 is controlled by the scheduler 9 so that the inputs of the AV selector 5 are selected and switched based on a schedule. An output from the AV selector 5 is input to the telopper 48, which performs necessary editing and processing of video signals (e.g.: superimposing characters on an image; shrinking an image and displaying characters in the emptied peripheral region of the image; or cutting away a portion of an image and inserting characters in the emptied portion). Thereafter, the resultant video signals are input to the video signal transmitter 49. The video signal transmitter 49 is responsible for signal amplification and cable loss compensation required for transmitting the NTSC video signal to the video signal receiver 113. The distance between the video signal transmitter 49 and the video signal receiver 113 is normally about 50 to 200 m.
FIG. 10 illustrates exemplary functional blocks of the video signal receiver 113. The video signal receiver 113 includes a Y/C separation circuit 21, a Y/U/V separation circuit 22, a sampling circuit 23, a clock generation circuit 52, a timing adjustment circuit 51, a scaling circuit 24 and an RGB conversion circuit 25. The Y/C separation circuit 21 separates an NTSC video signal into a color signal C and a luminance signal Y. The Y/U/V separation circuit 22 separates the color signal C into color difference signals R-Y (U) and B-Y (V).
The clock generation circuit 52 generates a reference clock signal of a frequency which is an integer multiple of the carrier frequency of the video signal, and provides the reference clock signal to the RGB conversion circuit 25, while the timing of the clock signal is adjusted by the timing adjustment circuit 51 so that a timed clock signal is provided to the sampling circuit 23 and the scaling circuit 24.
The separated signals are subjected to analog/digital conversion by the sampling circuit 23, scaled to the effective display region by the scaling circuit 24, and then converted to digital RGB signals by the RGB conversion circuit 25.
FIG. 11 illustrates exemplary functional blocks of the controller 14. The controller 14 includes a gamma conversion table 39, a multiplier 41 and a display division circuit 42. The output of the video signal receiver 113 is input to the controller 14, which performs an inverse gamma conversion by multiplying the input data by a coefficient from a gamma conversion table 39, so as to correct the characteristic difference between CRT and LED. Then, the display signal is divided by the display division circuit 42 into units corresponding to the LED panel displays 17 and converted into a format readable for the LED panel display 17.
The LED display section 8 in the display section 20 is illustrated in a simplified manner. In practice, the LED display section 8 includes a number of LED panel displays 17 arranged in rows and columns which are switched between the display state and the nondisplay state based on signals output from the controller 14. Each of light emitting blocks in the LED panel display 17 includes three LEDs respectively for the primary colors, R, G and B.
Table 1 below shows characteristics of a typical LED currently used in commercial applications.
TABLE 1 ______________________________________ Characteristics of typical LED used in commercial applications Chromaticity Peak Luminous coordinates wavelength efficiency Output Color X Y (nm) (lm/W) ______________________________________ R (red) 0.7 0.295 660 6.6 G (green) 0.17 0.7 520 10.0 B (blue) 0.15 0.07 475 3.6 YG (yellow green) 0.43 0.56 567 6.0 BG (blue green) 0.08 0.4 495 8.0 ______________________________________
FIG. 12 is another representation of Table 1 as a chromaticity diagram based on the CIE (Commission Internationale de l'Eclairage) standard xyz calorimetric system (hereinafter, referred to simply as "chromaticity diagram"). In FIG. 12, chromaticity coordinate values of a typical LED are represented by symbols such as R, G and B, while the respective display color ranges for the CRT and NTSC signals are represented by triangular regions.
However, when an NTSC video signal is displayed using the structure as illustrated in FIG. 9, the RGB signals obtained by separating the video signal are directly used for controlling the LED at three points, R, G and B (hereinafter, such control is referred to as "3 point control"). Thus, as is apparent from FIG. 12, there is a problem that the respective RGB chromaticity coordinate values do not conform with the display color ranges of CRT and NTSC signals.
The display color range of the structure illustrated in FIG. 9 can be adjusted to that of CRT in advance. However, in such a case, when it is desired to display graphic data of RGB signals such as those made on a personal computer, any chromaticity beyond the display color range of CRT cannot be represented.